A technique to reduce the permeability decrease in a chemical vapor deposition pore modification of a-alumina support by a SiH4/C2H2/NH3 reaction system has been proposed. SiC-Si3N4 nanoparticles were formed in the gas phase of a hot-wall reactor at a temperature of 1323 K and deposited in/on the pore of the support to form a modification layer. The structure of the particles was confirmed by Fourier transform infrared and high-resolution transmission electron microscopy techniques to be a composite of crystalline SiC dispersed in an amorphous phase containing Si3N4 The structure retarded the sintering between the deposited particles and resulted in a modification layer with a large porosity on the surface of the membrane. A single gas permeation measurement showed that a reduction of the average pore diameter from 0.30 to 0.21 mu m accompanied only an approximate 20% decrease in permeability: 31% reduction of the permeability decrease is achieved compared with the value predicted by a dense film deposition on the wall of the pore.